1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser used as a source for optical information processing, optical communications, optical recording and image forming. The present invention also relates to a method and apparatus for fabricating such a surface emitting semiconductor laser. More particularly, the present invention relates to a technique of accurately defining an aperture surrounded by a selectively oxidized portion of a current confinement region.
2. Description of the Related Art
Recently, there has been an increased demand for a surface emitting semiconductor laser capable of easily realizing an array of sources in the technical fields of optical communications and optical interconnections. Such a laser is also called vertical-cavity surface-emitting laser diode (VCSEL).
The surface emitting semiconductor laser is categorized into a proton injection type with a gain waveguide structure, and a selective oxidization type with a refractive ratio waveguide structure. Nowadays, the latter is getting the mainstream. Generally, the selective oxidization type semiconductor laser has a laser portion of a mesa structure (laser-use mesa). A current narrowing or confining region formed by a selectively oxidizing part of an AlAs layer or AlGaAs layer is formed in the vicinity of the active region of the mesa. The current confinement layer has an increased resistivity and a reduced refractive index. This results in an optical waveguide path.
The degree of dimensional accuracy of the non-oxidized region surrounded by an aperture formed in the current confinement layer and defined by the selectively oxidized region is a very important factor that determines the device performance. The threshold current of laser and the transverse oscillation mode greatly depend on the diameter of the aperture.
A proposal to solve the above problems is described in Japanese Unexamined Patent Publication No. 2001-93897. The proposal forms a linear stripe pattern that is formed on a substrate and has the same composition as that of a pattern of the laser portion having a mesa shape formed on the same substrate. The linear stripe pattern is used as a sample for monitoring. In an oxidization step, the degree of advance of the oxidization reaction on the monitor sample is monitored, and oxidization of the laser portion is controlled based on the monitored degree of advance. The proposal utilizes a phenomenon such that the reflectance of an AlAs layer in the monitor sample becomes higher as oxidization thereof advances. Light is projected onto the stripe-like monitor sample, and the reflectance thereof is monitored.
FIG. 13 is a graph of a relation between light projected onto the stripe-like monitor sample and its reflectance, and FIG. 14 is a graph of a method for controlling oxidization using the conventional monitor sample. Light of a selected wavelength is extracted from the light projected onto the monitor sample, and reflected light is monitored. As shown in FIG. 14, an AlAs layer of the monitor sample has reflectance values r1 and r2 when oxidization thereof starts (time t2) and ends (time t2), respectively. Oxidization of the current confinement layer is controlled by detecting the reflectance obtained at times t1 and t2 or its variation.
However, the proposal described in the above-mentioned publication has the following problems to be solved. The monitor-use sample has a width narrower than the diameter of the mesa of the laser portion (the outside diameter for a cylindrically-shaped mesa) and a stripe-like pattern formed on the substrate on which stripe lines are arranged with a constant period. The monitor sample is defined by anisotropic etching so that it can be simultaneously formed with the mesa of the laser portion. However, the monitor samples of laser devices have considerable dispersion of the line width due to the actual etching conditions. Sometimes, the sidewall (stripe edge) of the monitor sample is not vertical but inclined. When these faulty monitoring samples are subject to oxidization, optical diffraction may take place and prevent accurate measurement of variation from the reflectance r1.
FIG. 15 shows a graph of a relation between reflectance vs. oxidization time for the monitor sample. It is difficult to accurately identify the oxidization initiating time (circle of broken line in FIG. 15) from variation in the reflectance. Further, there is a dispersion of the stripe line width. Thus, the average reflectance r1 obtained from the stripe pattern fluctuates. This makes it difficult to accurately measure the reflectance r1 and detect the oxidization initiating time t1.
If tracking of the oxidized condition on the monitor sample includes error, it will be difficult to reproduce the aperture in the current confinement region as designed at the time of stopping the oxidization reaction. This degrades the production yield and prevents cost reduction. There is another disadvantage. After the step of oxidizing the current oxidization layer, an insulation film and a metal electrode are photolithographically formed. In these processes, the resist films may not be deposited on the substrate evenly. Resist is dropped on the substrate that is being rotated, the stripe pattern restricts movement of the resist and prevents the resist from smoothly moving towards the ends from the center of the substrate. This may prevent evenness in the film thickness of resist. The unevenness degrades the accuracy of an outgoing aperture formed in the metal electrode and affects alignment of the outgoing window with the aperture in the current confinement layer and the optical axis of the laser portion.